Grey casting is the term given to the method for casting grey iron. Grey iron is a pig or cast iron in which the carbon other than that of the perlite is present in the form of graphitic carbon. The important characteristic of grey iron as regards its use for engine blocks and the like is that it is machinable.
In the case of an automobile or truck, the cast engine components represent a significant portion of the gross vehicle weight. Because a reduced gross vehicle weight results in increased fuel economy, considerable attention has been given to reducing the weight of the cast engine components by reducing the thickness of the casting in certain areas, such as the cylinder block walls. The attempts which have been made in this regard have met with failure chiefly because as the wall thickness has been reduced, scrap loss has dramatically increased.
More specifically, among the important parameters affecting the lower limit on the thickness of the internal walls are the fluidity and solidification characteristics of the grey iron to be cast. In other words, the molten grey iron must be sufficiently fluid to flow into and fill relatively thin passages in the mold and have a sufficiently low solidification point so that the grey iron does not prematurely crystallize before the mold can be filled. As alluded to above, the difficulty with the processes which have heretofore been used to seek to make thin walled engine blocks has not been that the processes are incapable of making such engine blocks but rather that they are incapable of making them without a very heavy scrap loss.
Engine blocks of machinable grey cast iron are conventionally made by feeding into a cupola or other furnace the desired metallics and nonmetallics in the desired proportions such that molten cast iron of the desired chemistry is formed in the cupola, the molten metal from the cupola being conducted to a pouring station where it is poured into sand molds with the spaced supported sand cores therein. Since high quality machinable grey cast iron requires a high level of nucleation in the molten metal when it is poured, a particulate inoculant, generally ferrosilicon, is added to the molten metal just prior to the pouring operation so as to provide increased nucleation. Also, because high quality machinable grey cast iron requires that there be close control of the carbon equivalent (CE) of the metal and because the CE is a function of the silicon, carbon and phosphorous contents of the metal, the CE content of the metal prior to the inoculation is maintained lower than that desired to take into account the rise in the CE of the metal upon the addition of the inoculant.
It is, of course, necessary that the cupola or other furnace in which the molten cast iron is made have a capacity sufficient to supply the molten cast iron at the rate at which it is poured at the pouring station. To assure a continuous supply of the molten metal to the pouring station despite minor fluctuations in the output of the cupola, and to assure that there can be continued operation of the cupola despite brief interruptions or shutdowns at the pouring station, it is also conventional practice to feed the molten metal from the cupola into a small holding furnace, the metal for the pouring station being withdrawn from the holding furnace. Typically, the capacity of the holding furnace is sufficient to hold enough of the molten metal to be able to continue to supply it to the pouring station for about twenty minutes without receiving any molten metal from the cupola, and to be able to receive molten metal from the cupola for about ten minutes without feeding any to the pouring station. For example, if the metal is poured at the rate of sixty tons per hour then the cupola is operated at the same rate and the holding furnace has a capacity of about thirty tons but only contains about twenty tons during normal operation.
There is always at least some scrap loss in the manufacture of such engine blocks by reason of defects the engine blocks as cast. By far most of the defects occur in the thinnest wall portions of the castings, and the thinner the walls the greater the scrap loss. At present a scrap loss of about five percent in the casting operation is accepted by the industry as being nominal for engine blocks wherein the minimum wall thickness is about 0.180 inches. For engine blocks having substantially smaller wall thicknesses, for example, 0.150 inches, there is a dramatic increase in scrap loss, typically to as high as twenty-five percent. Such scrap losses are prohibitive as regards the manufacture of engine blocks for high production automobiles and trucks, and hence it is currently the practice in the automotive industry to design all high production engines to have engine block wall thicknesses of at least 0.180 inches. It is this limitation on the design of engine blocks that has become an ever increasing problem in the attainment of lesser gross vehicle weight.
The present invention solves this problem by providing a method whereby cast iron engine blocks can be made with wall thicknesses substantially less than are now used, without any increase in scrap loss.